Silicate-based deposits can occur in many industrial systems. For example, silicate-based deposits are a problem in some boilers, evaporators, heat exchangers and cooling coils. The presence of silica/silicate deposits can significantly reduce system thermal efficiency and productivity, increase operating/maintenance costs, and in some cases lead to equipment failure. Steam generators and evaporators are especially prone to silicate deposits due to operation at elevated temperatures, pH and increased cycles of concentration (COC).
In particular, silicate-based deposits are prevalent in produced water (steam assisted gravity drainage (SAGD), steam flood, etc.) plant unit operations. The type and structure of silica deposits can range from amorphous silica to highly complex metal silicates with hardness ions (primarily calcium/magnesium), aluminum, iron/alkali-metal ions (sodium, lithium, etc.). Amorphous silica, iron salts, carbonates, organic-based foulants and other chemicals may be incorporated into deposits.
For example, SAGD operations inject steam into geological formations to stimulate the production of bitumen or heavy hydrocarbon. Oil Sands deposits in Alberta, Canada represent an area where this process is extensively used. Pairs of horizontal wells are bored into the oil-containing formation. The upper well injects steam and the lower well which is positioned below the steam injection line, continuously extracts a complex emulsion. That emulsion contains bitumen and water. The emulsion is broken; the bitumen is sent for upgrading/refining, while the produced water (separated from the emulsion) is treated and reused as feedwater for the steam generators.
This SAGD process for producing bitumen results in large volumes of silica-laden water because oil sand formations contain large proportion of silica/silicate-bearing minerals (sand, clays, etc.) compared to the amount of bitumen present. Oil Sands “typically” consist of about 75% inorganic matter (including a large % of silica/silicates), 10% bitumen, 10% silt/clay and 5% water (Humphries, 2008). Large volumes of silica-laden produced water are returned from the 2-to-9 volumes of steam (cold water equivalent) injected per volume of bitumen recovered (Tristone Capital Inc. (2007). SAGD: Looking Beneath the Surface. Energy Investment Research, 4-5). Concentrations of silica returning to SAGD plant in the produced water tend to range from about 11-350 mg/L (Goodman, W., Godfrey M., Miller, T. (2010). Scale and Deposit Formation in Steam Assisted Gravity Drainage (SAGD) Facilities, International Water Conference in San Antonio, Tex., October 24-28, IWC-10-19).
There are two options for treating the returned produced water and supplemental makeup water for use as feedwater for steam generation. The first option is warm lime softening (WLS) and is the more traditional method for treating produced water. For silica reduction WLS is used followed with a weak acid cation (WAC) ion exchange for hardness removal. The treated water quality is poor relative to ABMA/ASME boiler feedwater standard guidelines. However, the use of Once-through steam generators (OTSG) mitigates the need for high purity water. In a preferred operation mode of the OTSG, the feedwater can have less than 8000 mg/L total dissolved solids (TDS) and near zero total hardness and the Silica (SiO2) specification is typically less than 50 mg/L. The WLS/Ion exchange process can achieve these requirements.
Evaporation technology (in particular mechanical vapor compression (MVC)) is the second and newer option of water treatment. The main reason for using evaporators to treat produced water is to achieve a very high quality of water so a conventional drum boiler can be used instead of OTSG. However, in some cases, evaporators are used to clean extremely dirty produced water along with other waste streams for use as feedwater in OTSG. As the industry looks to more and more recycled water, evaporators will play an important role in treating waste water for reuse. This can be accomplished because the evaporation technology is more robust and can be used on the more difficult to treat waste waters.
With evaporators, a high percentage of produced water is recovered as high quality boiler feedwater. High quality feedwater produced from evaporation enhances reliability of the steam generation equipment. The evaporator footprint is also significantly smaller than conventional WLS treatment.
Because of the nature of the water being treated, evaporators are likewise subject to deposition. Chemical treatment programs are used to minimize deposits, but evaporators can become fouled over time and cleaning is in order. Options for cleaning are chemical in-situ programs or mechanical.
As a result of significant silica/silicate deposit formation that can occur in unit operations such as evaporators, opportunities exist to improve system operations by using an effective in-situ chemical cleaning program. One option to deal with declining performance of Mechanical Vapor Compression (MVC) evaporators or evaporators in general due to scale deposits is to implement a chemical wash. Chemistries previously used are commodity acid or caustic which usually are not fully effective for dissolving silicate deposits. Those cleaners can be very hazardous to both equipment and personnel. If a chemical wash does not effectively dissolve tenacious deposits, then mechanical cleaning is performed. Mechanical cleaning is useful for removing flaky deposits but may only polish a more tenacious deposit without removing it and leading to a continued deposition of layers over time. Mechanical cleaning is very time consuming, expensive (e.g., for waste removal/labor costs), and can result in significant lost production. Thus, a need exists for a safe, novel chemistry used for evaporator washes to remove silica/silicate and organic-based deposits.